The present invention relates generally to rotary drum filters and more particularly to shower pipes for such filters.
Rotary drum filters are well known and are commonly used, for example, in the pulp- and paper-making industry for separating wood pulp from pulp slurry. Such filters include a rotary drum partially submerged in a tank of pulp slurry. The drum surface is conventionally covered by a filter screen. As the screen rotates through the pulp slurry, a vacuum is applied within a portion of the drum, collecting a wet mat of fibers from the slurry on the screen. As the screen emerges from the tank, slurry liquid or filtrate is drawn inwardly through the screen by the vacuum and discharged through suitable piping, thereby removing the liquid from the mat. Examples of such filters are disclosed generally in U.S. Pat. No. 4,276,169 to Browne, et al., U.S. Pat. No. 4,248,716 to LaValley, U.S. Pat. No. 4,370,231 to LaValley, and in the patents cited therein.
Conventionally, about half a dozen parallel shower pipes are angularly spaced around a segment of the drum extending from just above the surface of the slurry to the top of the drum, proceeding in the direction of drum rotation. These pipes extend axially of the drum and are supported adjacent the ends of the drum. Washing liquid is discharged in a spray from the shower pipes to wash the pulp fibers as the mat emerges from the slurry. The pipes are spaced a fixed radial distance from the filter screen. This distance is preferably constant along the length of the pipes so that spray intensity and distribution are substantially uniform all the way across the mat. Examples of such shower pipes are disclosed in U.S. Pat. Nos. 3,150,082 to Rich and 3,363,774 to Luthi.
One persistent problem in the design and manufacture of rotary drum filters for use in pulp manufacture is the mitigation of corrosion. This problem has been overcome in part by making as many components of the filters as possible of corrosion-resistant material, such as fiberglass reinforced plastic. However, prior attempts at making shower pipes of fiberglass reinforced plastic have suffered from several drawbacks.
One drawback is a tendency of the pipes to sag. Conventional drum filters are very long, often 20 feet or more. To span the length of the drum, the shower pipes must be even longer. Such pipes are ordinarily supported only at their ends and, during operation, are filled with washing liquid. Consequently, they must be extrmely stiff to minimize sagging between their ends. Prior shower pipes, constructed of fiberglass reinforced plastic alone and having a cylindrical cross section, are not sufficiently stiff to resist sagging.
Therefore, to increase stiffness in the vertical plane, it has been proposed to provide such pipes with a vertical fin or "stiff back" along one side. However, each pipe must be rotationally positioned so that its spray outlets or nozzles direct the spray against the mat at about the same angle of incidence. The rotational position thus varies with the angular position of each pipe around the drum. Applying a stiff back on all pipes in the same position relative to the spray outlets would defeat the purpose of the stiff back in the pipes that are rotationally positioned with stiff back approaching horizontal. Depending on pipe spacing, such positioning might also be precluded by interference between the stiff back of one pipe and an adjacent pipe. Hence, to be effective, this proposal would require making a different configuration pipe for each angular position around the drum.
Various shower pipe designs having steel angle members or stiffeners imbedded in the plastic at angular intervals around the pipe have also been tried, but still sag unacceptably. In one example, a 17 foot long cylindrical shower pipe, reinforced with three steel angle members, exhibited 15/32" sag when supporting a static load of 100 pounds midway between its ends. This is more sag than can be tolerated. Moreover, as this design of pipe is heated by the washing liquid, the steel stiffeners and fiberglass reinforced plastic expand differently, increasing both sag and the risk of cracking of the plastic or internal separation of the plastic and stiffeners due to thermal fatigue.
Another problem involves the spray pattern of washing liquid discharged from the shower pipes. It is desirable to wash the mat uniformly, necessitating continuous or overlapping spray coverage along the axial length of the drum. However, since the mat is typically thin and fragile, care must be taken to avoid tearing it with excessive localized spray intensity. Accordingly, a variety of different spraying arrangements have been proposed. One such arrangement uses a single row of spray holes spaced along the length of the pipe, together with some means for diffusing the spray from each hole to provide overlapping coverage. One form of diffusing means is a continuous lip or flange positioned along the row of holes and extending outwardly from the pipe to cause the spray from each hole to fan out. This approach is unsatisfactory because it concentrates too much spray pressure where overlapping adjacent fans of spray both strike the mat, often tearing the mat. Other forms of diffusing means include a whistle-type nozzle and a spoon-type diffuser positioned alongside each spray outlet. The latter forms of spray diffusers have also been tried in conjunction with double rows of longitudinally staggered holes. However, in practice, they do not provide sufficiently uniform spray coverage and are susceptible to plugging by fiber back-spattered from the mat into the spray outlets by the spray.
An additional disadvantage is the difficulty of making the foregoing shower pipe designs of fiberglass reinforced plastic using conventional manufacturing techniques. Heretofore, fiberglass reinforced plastic washer pipes have been formed on a cylindrical mandrel. Once a cylindrical pipe is formed, steel stiffeners are applied to its outer surface and overlain with additional fiberglass reinforced plastic material. Next, cylindrical spray holes are drilled in a row along a side of the pipe. Finally, a diffuser flange is positioned along the row of holes and secured to the pipe. This process produces a pipe having a rough and uneven outer surface. Such a surface precludes accurate positioning of the diffuser flange relative to the row of holes, thereby causing uneven diffusion of the spray. This process also renders virtually impossible the formation of complex shapes of spray holes or nozzles. And the resultant product has the functional drawbacks described above, namely, sagging and tearing the mat.
Many of the foregoing problems are eliminated by the shower pipe of the afore-mentioned copending U.S. application Ser. No. 464,023. Such shower pipe is made of fiberglass reinforced plastic and has a rectangular cross-section providing substantial rigidity against sagging between the ends of the pipe regardless of the rotational orientation of the pipe along a rotary drum filter. The pipe is stiffened by a pair of structural connecting flanges which extend along opposite sides of the pipe and provide rigidity to prevent the pipe from sagging. The pipe also includes reinforcing rods which extend along the interior corners of the pipe, also to provide rigidity against sagging.
Such pipes are preferably formed by molding them in channel-shaped half-sections which are subsequently joined together along their connecting flanges. Spray holes are machined in one of the half sections in two parallel rows in which the holes of one row are staggered from the holes of the other row. Continuous diffusion members extend along each row so as to diffuse the discharges of liquid from each row of holes into two different planes. In this way, overlapping spray coverage is obtained without any overconcentration of spray at any point on the mat. The spray holes have a constricting frustoconical shape to better diffuse or decolumnate the discharge of liquid onto the diffusion members.
Although the shower pipe of application Ser. No. 464,023 is a great improvement over known prior shower pipes, its integral essentially all fiberglass construction is laborious and expensive to manufacture, and requires laborious machining of each spray hole. Furthermore, while the continuous diffusion flanges along each row of spray holes are an improvement over prior hole-diffuser arrangements, the diffusion pattern they produce leaves room for more improvement.
Accordingly, there remains a need for a shower pipe which has the improved features of the pipe disclosed in application Ser. No. 464,023 but which is easier and less expensive to manufacture and has an improved spray diffusion pattern.
Therefore, a first object of the invention is to provide an improved shower pipe for rotary drum filters.
A second object of the invention is to provide an improved method of making such shower pipes.
A third object of the invention is to provide a shower pipe having an improved spray diffusion pattern to efficiently and effectively wash a mat without damaging it.
A fourth object is to provide an improved fiberglass reinforced plastic shower pipe which resists sag regardless of its rotational orientation and throughout its operational temperature range.
A fifth object is to provide a fiberglass reinforced plastic shower pipe which is easier and less expensive to manufacture than previous designs.
A sixth object is to provide an improved spray diffusion flange for a shower pipe.
A seventh object is to provide an improved method of providing spray holes and flanges on shower pipes.